Integrated circuits (ICs) may be severely damaged by electrostatic discharge (ESD) phenomena. An IC may be exposed to ESD from many sources. A major source of ESD exposure to ICs is from the human body, and is known as the Human Body Model (HBM) ESD source. A charge of about 0.2 micro-coulombs can be induced on a human body having a capacitance of 100 pF, leading to electrostatic potentials of 2 kV or greater. Any contact by a charged human body with a grounded object, such as a terminal of an IC, can result in a discharge for about 100 nano-seconds with peak currents of several amperes to the IC.
A second ESD model is the charged device model (CDM). Unlike the HBM ESD source, the CDM ESD source includes situations where the IC itself becomes charged and discharges to ground when any of its pins makes contact to a grounded conductive object. Thus, a CDM discharge requires only one IC pin to be contacted, whereas a HBM discharge requires at least two IC pins to be contacted. CDM pulses also have very fast rise times compared to the HBM ESD source.
Because of high electrostatic voltages resulting in large ESD currents on the one hand and low breakdown voltages of IC components on the other hand, the problem of ESD with IC components can be severe. Therefore, the terminals of an IC usually have an integrated protection device connected between the terminal and the internal circuits which allows the ESD current to be shunted to an alternative voltage source, e.g., ground, to clamp the induced overvoltage and protect the active internal circuits from damage.